Most types of prior art methods of measuring cardiovascular properties suffer from the problem that the performance of the measurements and the measurements themselves interfere strongly with the state of the patient, which may lead to erroneous results.
Additionally it is recognized that blood pressure often exhibit considerable variability over time. The newly published guideline by the UK health authorities is in consequence of these facts and the fact that diurnal variations are very important for proper diagnostics of hypertension (NICE clinical guideline 127, August 2011). It has also recently been shown that performing ambulatory blood pressure measurements is overall cost-effective (Lovibond K et al., Cost-effectiveness of options for the diagnosis of high blood pressure in primary care: a modelling study, Lancet. 2011 Oct. 1; 378(9798):1219-30).
Many prior art methods for blood pressure measurements require application of a counter pressure from an external pressure device, e.g. occlusive cuff or other pressure generating devices. These interfering methods generating an external pressure may have a significant impact on the person and the blood pressure. Blood pressure can for example be measured by an invasive pressure sensor, oscillometric or auscultatory tonometric. Blood pressure may also be derived from auxiliary parameters like pulse wave velocity. However, these methods require calibration against a known standard. These methods will inevitably affect the state of the patient, e.g. require surgery or use of an occlusive cuff applying an external pressure to the artery or require that the patient should be in a special position. Furthermore, it is well known that measurements of cardiovascular quantity performed at the office of a medical doctor or at a hospital often are quite erroneous and often consistently higher than when the patient measures it at home. This is normally referred to as the “white-coat syndrome”. However, merely the fact that a patient can feel that a measurement is performed will often have a psychological effect which results in a change of state of the patient.
Imaging methods provide information about the structure and dimensions of the limbs measured upon, i.e. the constituting organs and their respective tissues. Methods based on NMR or X-rays will in general have a resolution in space and time that is inadequate for measuring temporal variations on time scales comparable to or smaller than the time of a single pulse and accordingly a reliable determination of arterial distension cannot be based on such methods. Ultrasound may provide for adequate spatial and temporal resolution, but the method will often interfere with the state of the patient and thereby provide unreliable results. Optical coherence tomography can provide for the necessary spatial and temporal resolution, but here the penetration depth is very low. Hence, all the imaging modalities are unsuited for continuous ambulatory measurements and they are also very costly. However, the methods may provide relevant a priori information about the anatomy.
The stiffness (or elasticity) of arteries can be assessed by the pulse wave velocity method where the propagation velocity of pulses along the arteries is evaluated. The basic phenomenon is essentially acoustic. Propagation delays from e.g. the heart to the thighs, the wrist, or the foot are typically measured. However, the propagation length depends on the individual anatomy, which may exhibit considerable variations both in actual length and in vessel diameters. Also the pulse wave propagation velocity depends on the diameter of the artery and on the stiffness of the artery wall. These properties vary along the path from the heart to e.g. the wrist.
U.S. Pat. No. 6,443,906 describes a method and a device for continuously monitoring blood pressure. The method requires that the device is placed close to the wrist and comprises a sensor with a projecting portion—a plunger—which is pressed into the user's body to apply a force to an artery. The counter force on the opposite side of the artery is provided by the radial bone. The device will only function properly with such a positioning and the device exerts a force on the artery and accordingly the method will interfere with the state of the patient.
U.S. Pat. No. 5,309,916 describes a device for measuring blood pressure where the device includes a sensor arrangement which is attached to the exterior of a body and which is electrically conductively connected with an electronic circuit. The sensor arrangement and the circuit are configured to determine, in at least one measuring region of the body, a value which is a measure for a variable that changes periodically over time in the rhythm of the pulse beat, such as the flow velocity, flow quantity, the volume of the arterial blood, a cross-sectional dimension and/or the flow cross section area of an arterial blood vessel. The sensor and circuit further determine a value which is a measure for the pulse wave velocity. By linking the two values together and including at least one calibration value, at least one value that is characteristic for the blood pressure can be determined. A number of different measuring principles are mentioned such as measuring by light or ultrasonic radiation.
As clearly specified in U.S. Pat. No. 5,309,916, the method requires an individual calibration, i.e. the blood pressure of the specific patient should be measured by direct measurement e.g. using an inflatable cuff. Such individual calibration is both cumbersome and may lead to erroneous results. Furthermore it is not described how and which parameters should be related to the calibration measurement. A calibration measurement taking a blood pressure state of the patient, may likely not be reliable for application to correlate determinations of other blood pressure states, which means that a reliable calibration requires calibrating measurements of a large number of different blood pressure states of the same patient.
WO 2007/000164 discloses a method and an apparatus for non-interfering blood pressure measurements. The method and apparatus are based on capacitive sensing where the tissue cross-sections constitute most of the dielectric of the capacitor and where the capacitor forms part of a resonant circuit. However, since the conductivity of blood generally is very high, electrodes that are electrically isolated from the body are needed and calibration is needed. It is noted that the method exploits only the imaginary part of the impedance formed by the electrodes and the material separating the electrodes.
WO 2010/057495 discloses a method for combining distension measurement and pulse wave velocity to obtain a calibrated blood pressure, based on the capacitive sensing method as disclosed in WO 2007/000164. The application does not disclose a direct method for obtaining both the blood pressure variations and the absolute blood pressure.
US 2005/0283088 discloses a method for determining stroke volume from bioimpedance measurements involving the brachial artery but does not involve determining any of the quantities: Blood pressure, vascular stiffness or vascular compliance.
It is an object of the present invention to devise a method that allows for non-invasive determination of one or more cardiovascular quantities, such as blood pressure, wherein the determination does not require individual calibration and which method simultaneously results in highly reliable determinations.
It is a further object of the invention that the method can be performed in a simple manner e.g. by the patient or assistant that does not require special education but merely simple instructions.
These and other objects are obtained by the invention and embodiments thereof as defined in the claims and described herein below.